When comparing Sciter vs Flutter, the Slant community recommends Flutter for most people. In the question“What are the best cross-platform GUI toolkits?” Flutter is ranked 15th while Sciter is ranked 19th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Multi language
Support for C++, C#, Delphi, D, Go, Rust, Powerbuilder. See Go bindings on GitHub. The binding for C# on GitHub, SciterSharp does not seem to be free: in a commercial product you should acquire a commercial license.
Pro Lightweight
Only a single native DLL.
Pro Open source
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro A single codebase for iOS AND Android
Pro Fast
The developer's goal is to allow people to make apps running at 120 FPS.
Cons
Con Linux version is not very mature
The Linux version is missing HTML/CSS features when compared to the Windows version.
Con Not fully HTML5 compliant
Lacking HTML5 functionality and W3C standards: grabbing a library like JQuery or Bootstrap and use it in Sciter will not work.
Con Not WYSIWYG
Not WYSIWYG like WebForms or WPF.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers. BUT if you have to learn it ... it's a con.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.